BACKGROUND The development and progression of periodontitis are accelerated by various systemic conditions. The present study was designed to determine whether lactation affects alveolar bone loss in rat models of experimental periodontitis. METHODS Sixty-two female Wistar rats were bred with male rats and divided into three groups that were fed diets containing 0.9%, 0.3%, and 0.02% calcium. They were divided further into two subgroups of lactating and non-lactating animals. An elastic ring was placed around the neck of the right mandibular first molar to induce periodontitis (experimental side) on day 32 after mating. The left first molar was not fitted with an elastic ring (control side). After the lactation period, bone mineral density (BMD) was determined, and a histologic examination of the interdental alveolar bone was performed. RESULTS On the experimental and control sides, BMD decreased significantly according to the amount of calcium in the diet; however, the magnitude of this decrease was much greater in the lactating group. Histologic examination revealed that in lactating and non-lactating rats, the decrease in BMD was accompanied by a decrease in alveolar bone height on the experimental side, whereas similar results were not seen on the control side. CONCLUSIONS Lactation could be a risk factor for alveolar bone loss, especially under conditions of calcium insufficiency. Increased systemic demand for calcium and an insufficient supply of calcium might enhance the development of alveolar bone loss in periodontitis.